Police in the Swedish capital are to seek reinforcements after youths set cars ablaze and threw stones at police for a fifth night running.

Around 30 cars were set on fire in poorer neighbourhoods in northwestern and southwestern parts of Stockholm last night and rioters caused widespread damage to property, including schools.

"In terms of extent, it is a little less, a little quieter," police spokesman Kjell Lindgren said of the disturbances.

Eight people, mostly in their early 20s, had been detained during the night.

The violence appears to have been sparked by the death of a 69-year-old man who was shot by police in the suburb of Husby earlier this month.

Mr Lindgren said police were planning to request reinforcements from other areas to help deal with the rioting, upcoming football matches and the wedding of Princess Madeleine on 8 June.

He said the police needed to be prepared to maintain a heavy presence on the streets.

The unrest has led the United States embassy to warn US citizens this week not to go to areas hit by rioting.

"I can confirm we have sent out a Warden message," embassy spokeswoman Danielle Harms said, referring to alerts by the Department of State with safety or travel information.

Up to a third of young people, aged 16 to 29, in some of the most deprived areas of Sweden's big cities neither study nor have a job, a recent government study showed.